Different measures are typically used in attempts to control or alleviate NO.sub.x, namely the so-called primary and secondary measures. In the primary measures, a number of combustion modifications may be made to reduce NO.sub.x emissions. There are many different possibilities of boiler modifications, such as low excess air, biased burner firing, over-fire air, flue gas recirculation, and the like, as well as any combination of these and other primary modifications.
When the limits of NO.sub.x emissions cannot be met with combustion control only, flue gas treatments systems, so-called secondary, or post-combustion, measures, may have to be implemented. Among these secondary measures, the dominant method in use is selective catalytic reduction (SCR). In many instances, an SCR installation may provide NO.sub.x reduction as high as 80-90%.
In the selective catalytic reduction method the NOx concentration in the flue gas is reduced through injection of ammonia and passing the flue gas through a catalyst. The role of the catalyst and the reaction mechanism results in reduction products of nitrogen and water. The reaction is selective, which means the oxidation of ammonia and sulfur dioxide should not occur.
The efficiency of SCR system is dependent upon several factors, such as NO.sub.x concentration at the inlet of the catalyst, the flue gas temperature and composition, the ratio of ammonia injection to NOx concentration, and catalyst size and properties such as space velocity, catalyst activity, and active area. Furthermore, a number of other factors, such as the chemical formulation of the catalyst, the type and chemical composition of the fuel being burned, add-mixtures being used for boiler conditioning or deslagging requirements, ammonia dispensing techniques, and the like, also have very significant effects on the efficiency and life of the catalyst being used in any SCR process.
Looking further at the main characteristics of the design of an SCR system in, for example, a fossil fuel burning power generation plant, as well as problems associated with such designs:
A. Space velocity is considered to be a crucial design parameter in an SCR reactor for it is a measure of the residence time for the flue gas mixture (at selected temperature and pressure), within the volume of the catalyst. Calculation of the required space velocity for a given application takes the following factors into account: required efficiency, catalyst activity, temperature, permissible ammonia slip, flue gas analysis and dust composition. Without the means for the quick, efficient and inexpensive field verification of the theoretical predictions of required space velocity, very disappointing operational problems, with resultant potential deleterious environmental impacts, were all too often the norm, rather than the exception, when relying upon theory only. PA1 B. The selectivity of the catalyst defines the extent to which the desired reactions occur. A decrease in selectivity allows unwanted reactions such as the oxidation of sulphur dioxide to sulphur trioxide to occur. The oxidation of sulphur dioxide to sulphur trioxide depends mainly on the properties of the catalyst and the flue gas temperature. The actual amount of sulphur trioxide produced depends also on the original concentration of sulphur dioxide in the flue gas. More active catalysts with a lower specific volume lead to a higher rate of sulphur oxidation. The reaction, however, is temperature dependent. Most SCR plants have a guaranteed value for the maximum permitted sulphur dioxide oxidation at a specific flue gas temperature. The ability to make this guarantee is dependent, to a large extent, upon modeling and empirical assumptions, and it is not until the invention described herein, that these assumptions can be rapidly, economically and accurately proved on site, under actual operating conditions. PA1 C. Unreacted ammonia used in an SCR process will react with sulphur trioxide in the presence of water. The result depends on the concentration of ammonia and sulphur trioxide and flue gas temperature. This unwanted product is ammonium bisulfate (NH.sub.4 HSO.sub.4), a sticky compound which can cause fouling and blocking of equipment downstream in the flue gas flow. This fouling problem may be particularly acute at the cold and intermediate layers of air preheaters. This problem may also be present in known SNCR type applications, depending upon the degree of ammonia slip, natural SO.sub.3 concentration in the flue gas flow the temperature of the flue gas, as it passes through the air preheater. The importance of this latter statement will become more apparent after a reading of the following detailed description. PA1 D. Location of the catalyst in the duct may be particularly critical. For example, in a high dust applications, the catalyst is usually placed where the flue gas temperature is right for most types of catalyst, between the economizer and the air preheater. The flue gas passing through the catalyst contains most of the fly ash and sulphur oxides from combustion. Depending upon design criteria, this can cause degradation of the catalyst leading to premature and/or permanent decrease in NOx reduction efficiency. Because of the problems associated with high dust locations, along with the benefits often present in these locations, several changes have been considered in the past when retrofitting a high dust location SCR; however, in many of these instances, such changes were made with only empirical input, or with actual full scale installations. By means of the present invention, as will be described hereinafter, the designer will have many choices to investigate and will be able to quickly, economically and accurately obtain real time and/or accelerated results in actual field conditions. PA1 E. In very rare occasions, an SCR system may be located in a low dust condition situated after a hot gas precipitator and before the air preheater. The flue gas reaching the catalyst is almost dust free, but contains sulfur dioxide, with the attendant effects discussed herinabove insolar as the formation of NH.sub.4 HSO.sub.4. Hot side precipitators have lost much favor in the United States; however, they are still being installed in Japan. The present invention will permit the field testing of the validity of considering SCR at a low dust condition following a hot side precipitator. PA1 F. Tail end SCR systems have the catalyst situated in the end of the chain of flue gas cleaning equipment, or after the desulphurization plant. The flue gases reaching the catalyst therefore contain only small amounts of sulphur oxides and particulates. The flue gas temperature after the desulphurization plant is too low for most types of catalyst, so reheating is needed. This sort of facility is very rare; however, even if used, the simple testing method and apparatus of the present invention will be invaluable in choosing catalyst, determining the amount of reheating necessary, and verifying other design considerations. PA1 G. Catalysts, for example ones used for coal-fired plants, are designed mainly for parallel flow and can be characterized as one of two primary types, plates or extruded, in any of several configurations, including honeycomb (both of which are referred to hereinafter as expanded surfaces, as are other types which may be envisioned being used in these types of applications, for example molded surfaces of a composite material). The selection of the catalyst geometry is critical depending upon the operating conditions, the type of coal being burned, the number of catalyst layers and the like. Heretofore this selection has been mathematically simulated, often supplemented with mock tests, or physical scale modeling. By means of this invention, the performance and longevity of a number of catalyst structures can be economically, efficiently and accurately tested side by side, in actual field conditions, either in real time, or accelerated. PA1 H. The chemical composition of the catalyst used in SCR systems of the type discussed is determined by a number of criteria, for example: the flue gas temperature; NO.sub.x reduction required; permissible ammonia slip; permissible oxidation of sulphur dioxide; concentration of pollutants in the inlet flue gas and homogeneity of the flue gas flow and guaranteed lifetime of the catalyst. These criteria are perhaps the most onerous to determine accurately in theoretically differing circumstances, and often lead the decision maker to have to grant a contract to the catalyst manufacturer willing to "gamble" the most, rather than the one who has the longest lasting, least expensive, and/or most efficient catalyst. By means of the present invention, claims of various manufacturers can be readily determined, in side by side actual field tests, at a very attractive cost/benefit ratio and, if desired, with accelerated results. PA1 Performance Improvement--the actual operational testing afforded by the present invention will aid in verifying and optimizing efficiency vis a vis effect of flue gas and fly ash compositions, effect of distribution (flow, temperature, ammonia, NO.sub.x), simultaneous activity under real conditions of a number of factors such as location, sizing and type of substrate, variable loading cycling, and the like. PA1 Life Expectancy--the testing procedure of the present invention will readily identify potential seriousness, and/or timing of catalyst masking, catalyst poisoning, catalyst erosion, effect of water/vapor, problems with trace elements, cleanability, effect of additives, ambient problems (i.e. salt in the air), results of attempts of optimization of other portions of the plant, and the like. PA1 Balance of Plant Impact--the testing taught by the present invention will greatly assist in developing economic schemes to alleviate pluggage, ammonia slip from SCR and/or SNCR, with or without SCR, the ability to use and the effectiveness of ammonia destruction catalyst, air preheater effects, predictions on the ability to market the ash, and the like. PA1 Modeling--Modeling results will become much more reliable, nomographs may be developed in certain circumstances, particulate projectory patterns will be simpler to model and estimate, math assumptions can be validated to a much greater degree of accuracy, and the like.